A servo control apparatus is an apparatus that uses an actuator to perform control such that the position of a driven body, which is a subject to be driven, agrees with a command position while the position of the driven body is detected using a position detector. A multi-degree-of-freedom machine apparatus having a movable range on a two-dimensional plane or in a three-dimensional space, such as a numerical control machine tool, an industrial machine, a robot, or a transfer machine, includes servo control apparatuses, each of which is referred to as a single-degree-of-freedom axis. In such a machine apparatus, actuators attached to the shafts each control a driven body on a corresponding one of the axes so as to drive the driven body. The machine apparatus combines their axial motions to achieve multi-degree-of-freedom machine motion.
Servo control that is performed to cause a motion trajectory to follow a command trajectory or a command path accurately is referred to as trajectory control or contour motion control. If an extraneous disturbance factor, such as friction caused during the trajectory control or a machine structure vibration caused during the trajectory control, causes the servo control apparatus each axis to have a response error, the motion trajectory of the driven body deviates from the command trajectory, and thereby a trajectory error is caused.
In the case of a numerical control machine tool that transfers a motion of each axis onto a workpiece to be machined so as to create a form therein using a cutting tool, a trajectory error of a few tens of micrometers may be deemed as a machining defect. As a countermeasure, feedback control may be performed using a position detector; even so, the position detector cannot detect the position of the subject to be controlled accurately if the installation position of the position detector does not agree with the position of the driven body or if the position of the driven body does not agree with the position of the true subject to be controlled, i.e., the tool or the workpiece. Thus, a trajectory error is caused in the motion trajectory of a subject to be controlled.
If a trajectory error is caused in a motion trajectory, the motion trajectory of the machine is measured to identify a cause of the problem; then, a design change may be made to the machine or various parameters of the servo control apparatuses may be adjusted. In order to adjust the various parameters, it is necessary to measure the amount of deviation of the motion trajectory from the command trajectory with an accuracy of a few micrometers to a few hundreds of micrometers. In Patent Literatures 1, 2, and 3, methods of measuring and detecting a machine motion trajectory are disclosed.
A motion accuracy test method disclosed in Patent Literature 1 measures a relative motion trajectory between two steel balls coupled by a displacement detector by reading a displacement caused in a relative distance between the two steel s when an arc motion is performed in such a manner that the relative distance between the two balls is kept constant. This measuring method, which is called the double ball bar method, has achieved widespread use. In a typical double ball bar method, a trajectory error caused during an arc motion can be measured with an accuracy of a few micrometers by measuring a displacement of the bar in motion using a displacement sensor with the two steel balls mechanically connected.
A numerical control apparatus disclosed in Patent Literature 2 detects the speed or acceleration of the subject to be controlled and compares the detected speed or acceleration to a planned speed or acceleration to detect the occurrence of a trajectory error. In Patent Literature 2, a method of inhibiting a trajectory error by feeding back the detected speed or acceleration component of the trajectory error is also disclosed.
In Patent Literature 3, a method of estimating a motion trajectory of a driven body by performing second order integration on the acceleration of the driven body observed when a sine wave motion is performed using one or more axes is disclosed. In Patent Literature 3, a method of estimating a motion trajectory with high accuracy by changing a sensitivity factor so as to prevent an error caused when the integration is performed from exceeding a threshold value is also disclosed.